1. Field of the Invention
A method for selecting a modulation and coding rate scheme (MCS) in a multiple-in multiple-out (MIMO) system is provided.
2. Related Art
The design of communication systems for wireless local area networks (WLANs) is based on a family of standards described in IEEE 802.11. To address multipath and, more particularly, the fading caused by multipath (wherein objects in the environment can reflect a transmitted wireless signal) and other conditions, a wireless system can employ various techniques. One such technique is configuring the WLAN as a multiple-input multiple-output (MIMO) system.
FIG. 1 illustrates a simplified MIMO system 100, which can transmit on multiple antennas simultaneously and receive on multiple antennas simultaneously. Specifically, a transmitter (Tx) 101 can transmit signals simultaneously from antenna 102A (using a transmitter chain 103A) and from antenna 102B (using a transmitter chain 103B). Similarly, a receiver (Rx) 104 can receive signals simultaneously from antenna 105A (using a receiver chain 106A) and from antenna 105B (using a receiver chain 106B). The use of multiple antennas, depending on the specific implementation, can either extend the range or increase the data rate at a given range. Note that multiple streams of data can also be sent using the multiple antennas. MIMO system 100 can also advantageously minimize the differences in signal to noise ratio (SNR) across different frequency bins.
In the IEEE 802.11a/g standards, the sub-carriers of the channel associated with an OFDM-encoded signal may be modulated using a different modulation. Exemplary modulations include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16 quadrature amplitude modulation (QAM), 64 QAM, etc.
Adding redundancy to the signal using convolutional coding, simply referenced herein as coding, can also be used to increase the probability of accurate signal reception. The coding rate is a ratio of the number of transmitted bits to the number of actual data bits to be transmitted. For example, a coding rate of ½ refers to transmitting 2 bits for every 1 actual data bit. The tightest coding rate is currently ⅚ in which 6 bits are transmitted for every 5 data bits.
Notably, in conformance with the 802.11 standards, each modulation can have a predetermined coding rate. For example, Table 1 lists various modulations and their associated coding rates as provided in the 802.11a 1999 standard.
TABLE 1MODULATION AND CODING RATESCodedDataCoded bitsbits perbits perData rateCodingperOFDMOFDM(Mbits/s)Modulationratesubcarriersymbolsymbol6BPSK½148249BPSK¾1483612QPSK½2964818QPSK¾296722416-QAM½4192963616-QAM¾41921444864-QAM⅔62881925464-QAM¾6288216
In a standard MIMO system, a single or multiple data stream can be transmitted. The number of streams, the modulation, and coding rate scheme (MCS) can be set based on the channel condition. An overall indicator of the channel condition is the received signal strength indicator (RSSI). This RSSI can be used to compute a bit error rate, thereby allowing at least an informed MCS selection.
Note that random bit errors can occur because of white Gaussian noise. These bit errors can be easily corrected using convolutional coding. Unfortunately, the bit error rate is only one element of the total packet error rate performance. Specifically, the bit error rate does not include burst errors. Burst errors can occur because of fading or strong interfering signals. These burst errors include a large number of errors within a short period of time and therefore cannot be easily corrected using convolutional coding.
Thus, a standard RSSI measurement is not sufficient to accurately measure how well a packet of information can be successfully transmitted using a given MCS. Therefore, a need arises for an accurate measure of total error rate performance, wherein using the accurate measure of total error rate performance can then result in an optimized MCS selection.